The present invention relates generally to electrodes and more particularly to prosthesis electrodes for stimulating nerve tissue or muscular tissue in a body.
For therapeutic purposes it is sometimes desirable to electrically stimulate or excite body tissue. This is done with an electrode implanted in the body next to the body tissue which is to be excited. Problems arise from this practice.
The more conventional electrode was typically composed of metal, such as platinum or tantalum, and the electrical charge is carried by electrons in the metal and by ions in the fluids of the body tissue. When direct current is passed through the electrode to excite the body tissue, there is an electrochemical reaction at the interface between the metal electrode and the body tissue, resulting in degradation or corrosion of the electrode and in the generation of reaction products in the body tissue. These reaction products may be toxic, especially if allowed to build up or accumulate in the body tissue.
In an attempt to avoid the undesirable results arising from the use of direct current, use has been made of biphased, balanced electric waveforms. These comprise alternating positive and negative pulses which result in no direct current over a full cycle and a zero net charge transfer. This approach theoretically reduces electrochemical damage by reversing, in the second part of the waveform, some of the electrochemical reactions occurring during the first part. However, even under these conditions, harmful amounts of reaction products may still accumulate over a period of time, and it is, therefore, desirable that electrochemical reactions at the electrode-tissue interface be avoided altogether. This can be done by limiting the charge passed on a single pulse to that required to charge the so-called electrical double layer, a charge below that which would cause electrochemical reactions. The latter occur when the charge exceeds the breakdown voltage for the capacitance inherently formed between the metal electrode and the body tissue fluid (a natural electrolyte).
Prosthesis electrodes are relatively small, and the amount of charge which can be transferred without electrochemical reaction is proportional to the surface area of the electrode. Accordingly, the surface area imposes limitations on the amount of charge which can be stored. There have been efforts to increase the effective surface area of the all-metal electrode by roughening it, e.g. by peening, etching or platinizing (depositing, by electroplating, a layer of small platinum particles).
Another approach to avoiding the undesirable results arising from the use of all-metal electrodes was the use of capacitor electrodes in which the metal was coated with a thin layer of dielectric material, such as an organic polymer, thereby completely insulating the metal from the body tissue. A biphased, balanced waveform is applied to this electrode.
In general, the capacitance of such electrodes is too small to be practical. This is because the charge must be sufficiently high to excite or stimulate the body tissue. However, if, in order to achieve the stimulus charge, the breakdown voltage of the capacitor is exceeded, there will be a flow of current from the electrode to the body tissue producing the undesired electrochemical reactions.
One form of prior art prothesis electrode of the capacitor type is composed of tantalum coated with a thin layer of tantalum pentoxide as the dielectric material. However, the latter is a good insulator for tantalum for only one polarity of the applied voltage. Therefore, the stimulating electrode would always have to be positive with respect to the body tissue to prevent reduction of the tantalum pentoxide coating and generation of hydrogen gas, which is undesirable. In addition, the amount of charge provided by this type of electrode is too small for many prosthesis applications.
The prosthesis electrode described in the related application provides a high charge transfer capability and long term stability, and it avoids the buildup of toxic reaction products in the body tissue. This prosthesis electrode comprises a metal tip covered by a thin layer of non-metallic coating in turn covered by an ion transfer membrane composed of a single layer of material which is non-toxic to body tissue. The non-metallic coating is preferably composed of a cation corresponding to the metal in the electrode tip and an anion corresponding to an anion in the fluid of the body tissue. Alternatively, the anion in the coating is one which will not form a toxic reaction product when combined with a cation in the body tissue fluid. With either such anion alternative, the membrane is an anion transfer membrane which, during pulsing, inherently permits movement therethrough of the anion in the coating while preventing movement therethrough of the cation in the coating.
In another embodiment of the prosthesis electrode of the related application, the membrane may be a cation transfer membrane, and, in such a case, the cation in the non-metallic coating (and the metal of which the electrode tip is composed) is a cation which, when combined with an anion in the body tissue fluid, produces a compound which is non-toxic or will not build up in toxic quantities in the body tissue (e.g. because it is so insoluble in the body tissue fluid).
In both embodiments of the electrode of the related application, positive charge is passed through the electrode by means of an electrochemical reaction, in the coating, of the metal of the tip to form additional coating or to change the valence state of the cation in the coating. The reverse occurs for passage of negative charge. The ion transfer membrane prevents or inhibits movement into the body tissue fluid of soluble ions of the coating which may have toxicity to the body tissue.
As a result, there may be applied to the electrode of the related application whatever charge is necessary to stimulate the body tissue including the high charge necessary to stimulate optical or auditory nerves, and this may be done without concern about exceeding a breakdown voltage of the capacitor electrode; and there is no need for special manufacturing steps to increase the effective surface area of the electrode to accommodate a large charge without exceeding a breakdown voltage.
The prosthesis electrode of the related application has the capability of stimulating body tissue with a charge or current density which may be from 2 to 10 times higher than that of a more conventional platinum or tantalum electrode. However, there is a problem which arises when such high current or charge densities are used, a problem not recognized with the lower current and charge densities provided by the more conventional platinum and tantalum electrodes.
This problem arises from the presence of sodium chloride in body tissue fluid. At the high current and charge densities available with the prosthesis electrode of the related application, the concentration of the sodium chloride in the body tissue adjacent the electrode will change during a biphasic pulse. More specifically, with a positive pulse of current, chloride ions migrate to the electrode and sodium ions migrate away, resulting in a deficiency of sodium chloride in solution in the tissue at the electrode-tissue interface. A negative pulse gives a corresponding increase in concentration of sodium chloride in the tissue at the electrode-tissue interface. Such changes in the concentration of sodium chloride in the body tissue are undesirable.